Chemokine receptors, a subclass of the G-protein coupled receptors (GPCRs) are expressed on the surface of T-cells and other leukocytes. The interaction of chemokine receptors with their ligands plays an important role in the migration of leukocytes to sites of inflammation (A. D. Luster, New Engl. J. Med., 1998, 338, 436). The chemokine receptor CXCR3 is preferentially expressed on T helper (Th1) cells but is also found on natural killer cells and subsets of dendritic cells. Three major chemokine ligands for CXCR3 have been identified: Mig (Monokine Induced by γ-IFN/CXCL9), IP-10 (γ-interferon inducible protein) and I-TAC (IFN-Inducible T Cell α Chemoattractant/CXCR11) (K. E. Cole et al., J. Exp. Med., 1998, 187, 2009; Y. Weng et al., J. Biol. Chem., 1998, 273, 18288).
Histological evaluations of numerous inflammatory lesions, including those from patients with multiple sclerosis (T. L. Sorenson et al., J. Clin. Invest., 1999, 103, 807), rheumatoid arthritis (S. Qin et al., J. Clin. Invest., 1998, 101, 746), psoriasis (J. Flier et al., J. Pathol., 2001, 194, 398) and inflammatory bowel disease (Y. H. Yuan et al., Inflamm. Bowel Dis., 2001, 7, 281) have shown elevated expression of CXCR3 ligands accompanied by an increased frequency of T cells bearing CXCR3. This is in marked contrast to what is found in most normal tissues, where expression of CXCR3 and its ligands is extremely low. This correlative evidence suggests a role of CXCR3 in Th1-mediated chronic inflammation.
Studies with CXCR3 and IP-10 deficient mice also suggest a role for CXCR3 and IP-10 in Th1 mediated disease. For example, in one study CXCR3 −/− mice showed significant resistance to allograft rejection (W. W. Hancock et al., J. Exp. Med., 2000, 192, 1515). In another study, IP-10 deficient mice showed protection against the development of colitis (U. P. Singh et al., J. Immunol., 2003, 171, 1401). Further evidence of a role for CXCR3 and IP-10 as mediators of disease is provided by studies utilizing blocking antibodies. For example in a rat model of adjuvant induced arthritis (I. Salomon et al., J. Immunol., 2002, 169, 2685) a DNA vaccine approach to overexpress self IP-10 was used to induce the production of self-IP-10 antibodies. These Abs are specific for IP-10 and do not cross react with other proinflammatory cytokines or chemokines including Mig and I-TAC. Pretreatment with this vaccine protected rats from the development of severe arthritis and reduced the time to remission of symptoms. In addition, affinity purified anti-IP-10 from vaccinated rats could therapeutically transfer protection to newly diseased rats. In another study, this vaccine approach was successful in suppressing disease in a mouse model of multiple sclerosis (G. Wildbaum et al., J. Immunol., 2002, 168, 5885).
In a study of pulmonary inflammation, (N. Li et al., Acta Pharmacol. Sinica, 2008, 29, 14) CXCR3 knockout mice showed alleviated inflammation compared to wild type mice in cigarette smoke induced pulmonary injury as well as lower influx of inflammatory T cells. Similarly, in a model of nephrotoxic nephritis, CXCR3 knockout mice showed reduced influx of T cells, less severe nephritis and improved renal function compared to wild type mice (U. Panzer et al., J. Am. Soc. Nephrol., 2007, 18, 2071). Thus CXCR3 may play a role in inflammatory pulmonary diseases such as COPD and inflammatory kidney disease.
Studies such as those cited above suggest that inhibitors of CXCR3 may be useful in treating inflammatory and autoimmune diseases in which CXCR3-mediated cellular recruitment plays a role, including multiple sclerosis, psoriasis, rheumatoid arthritis, inflammatory bowel disease, COPD and kidney disease.
Recent work has also implicated CXCR3 in the pathogenesis of atherosclerosis. In one study (F. Mach et al., J. Clin. Invest., 1999, 104, 1041) CXCR3 was found expressed in all T lymphocytes within human atherosclerotic lesions. The ligands IP10, Mig and I-TAC were all found within lesion-associated cells including endothelial and smooth muscle cells (Mig and I-TAC) and macrophages (IP10), suggesting these ligands play a role in recruitment of activated T lymphocytes within vascular wall lesions in atherogenesis. Left untreated and allowed to progress, atherosclerosis can result in narrowing of the lumen of the artery and plaque rupture which can lead to coronary heart disease, myocardial infarction and stroke (J. Sanz and Z. A. Fayad, Nature, 2008, 451, 953).
Further evidence has come from genetic deletion studies in mice. CXCR3 deletion on an ApoE−/− background resulted in a significant reduction in atherosclerotic lesion formation following ten weeks on a high cholesterol diet (N. R. Veillard et al., Circulation, 2005, 112, 870). Moreover, deletion of the CXCR3 ligand, IP-10 on an ApoE−/− background similarly reduced atherosclerotic lesion load (E. Heller et al., Circulation, 2006, 113, 2301). More recently, NBI-74330 a CXCR3 antagonist was dosed prophylactically in a LDL receptor knockout model. Similar to the CXCR3 deletion studies in the ApoE−/− results, NBI-74330 significantly attenuated atherosclerotic lesion formation (E. J. A. van Wanrooij et al., Arterioscler. Thromb. Vasc. Biol., 2008, 28, 251).
As a result of studies such as those cited above implicating the interaction of CXCR3 and its ligands in the etiology of various inflammatory and autoimmune diseases as well as atherosclerosis, considerable effort has been directed towards discovering antagonists of this interaction. A number of inhibitors have been reported in the scientific literature, including small molecule antagonists, antibodies and modified ligands (see for example J. C. Medina et al., Ann. Rep. Med. Chem., 2005, 40, 215). However, to date, no CXCR3 antagonist has been approved as a marketed drug.